The market for personal portable electronic devices, for example, cell phones, laptop computers, personal digital assistants (PDAs), digital cameras, and music playback devices (MP3), is very competitive. Manufacturers, distributors, service providers, and third party providers have all attempted to find features that appeal to the consumer. For example, manufacturers are constantly improving their product with each model in the hopes it will appeal to the consumer more than a competitor's product. Battery life is one area in which improvements are sought.
Rechargeable batteries are currently the primary power source for cell phones and various other portable electronic devices. The energy stored in the batteries is limited. It is determined by the energy density (Wh/L) of the storage material, its chemistry, and the volume of the battery. For example, for a typical Li ion cell phone battery with a 250 Wh/L energy density, a 10 cc battery would store 2.5 Wh of energy. Depending upon the usage, the energy could last for a few hours to a few days. Recharging often requires access to an electrical outlet. The limited amount of stored energy and the frequent recharging are major inconveniences associated with batteries. Accordingly, there is a need for a longer lasting, easily recharging solution for cell phone power sources. One approach to fulfill this need is to have a hybrid power source with a rechargeable battery and a method to trickle charge the battery. Important considerations for an energy conversion device to recharge the battery include power density, size, and the efficiency of energy conversion.
Energy harvesting methods such as solar cells, thermoelectric generators using a temperature gradient, and mechanical/kinetic generators using mechanical motion are very attractive power sources to trickle charge a battery. However, the energy generated by these methods is often small, usually only a few milliwatts to approximately a few hundred milliwatts depending on size, efficiency, nature of the energy source, etc. In the regime of interest, namely, a few hundred milliwatts to a few watts, this dictates that a sizeable volume or area is required to generate sufficient power for trickle charge. Such methods include coupling the battery to a solar panel (photovoltaic cell). See for example, U.S. Pat. No. 5,898,932 issued on 27 Apr. 1999.
Accordingly, it is desirable to provide an apparatus for efficiently charging a battery of a portable electronic device. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.